1. Field of the Invention
This invention relates to a probe assembly (or a potential detector) for an apparatus for measuring ionic activity. In the present invention, "an apparatus for measuring ionic activity" refers to an apparatus (i.e. an electrolyte analyzer) for potentiometrically measuring the ionic activities of electrolyte ions contained in aqueous liquid samples, for example, body fluids (e.g. whole blood, blood plasma, blood serum, and urine), liquor, and drinking water, and of a product formed by the reaction among the components contained in a liquid.
More particularly, this invention relates to an improvement in a probe assembly having at least two pairs of probes which come into contact with terminals of at least two pairs of ion-selective solid state electrodes of an ionic activity measuring device, each of said pairs of electrodes having an ion-selective layer sensitive to a predetermined specific ion, so as to measure the potential difference generated between each of said electrodes of each pairs of electrodes corresponding to ionic activity difference existing as to a specific ion between reference and sample solutions spotted on said ionic activity measuring device, the improvement being to function to prevent potential deviation resulting from cross talk interference between the pairs of electrodes and to minimize the influence of electrical noise on the measured potential.
2. Description of the Prior Art
Measurement of the ionic activity of an electrolyte ion, for example, Na.sup.+, K.sup.+, Cl.sup.-, Ca.sup.2+, HCO.sub.3.sup.- -(or CO.sub.3.sup.2-), or the like, has significant importance in clinical chemistry tests. As the main methods for measuring ionic activity, there are known such techniques as flame analysis in which a sample solution is injected into a flame to determine the ionic activity on the basis of the wavelength of the light thus generated, coulometry in which electrolysis is conducted in the sample solution to determine the ionic activity on the basis of the quantity of electricity produced, and potentiometry in which an ion-selective electrode is used to determine the ionic activity on the basis of the potential generated in proportion to the logarithm of the ionic activity of a specific ion contained in a sample solution supplied to said electrode. Among these methods, potentiometry has attracted much attention in recent years, since it is safer, quicker, and more convenient than flame analysis or coulometry.
In the potentiometric measurement of ionic activity, an ionic activity measuring device having ion-selective electrodes is utilized. As such a device, a dry slide-type device having a pair of film-like planar ion-selective electrodes is disclosed (for example, by Japanese unexamined Patent publication No. 52(1977)-142584, and U.S. Pat. Nos. 4,053,381 and 4,437,970). The basic structure of this ionic activity measuring device is that of a slide comprising at least one pair of solid state electrodes each having an outermost ion-selective layer and a porous bridge which can promote capillary action between a pair of said ion-selective layers. By spotting a reference solution on one layer of said pair of ion-selective layers and a sample solution on the other, and then measuring the potential difference between the electrodes, the ionic activity of a specific ion contained in the sample solution can be determined. Such a device having at least two pairs of ion-selective solid state electrodes is particularly preferable since it enables different kinds of ions to be measured simultaneously or successively.
Using the foregoing ionic activity measuring device, spotting of reference and sample solutions, measurement of potential difference and conversion of the measured potential difference into ionic activity are conducted in the apparatus for measuring ionic activity disclosed in Japanese patent application Ser. No. 59(1984)-12796.
In this apparatus, at least one pair of probes come into contact with terminals of at least one pair of ion-selective electrodes of the ionic activity measuring device so as to output the potential difference between the ion-selective electrodes to a measuring circuit. An example of a probe assembly having such a pair of probes is disclosed in Japanese unexamined patent publication No. 59(1984)-188550. By using such a probe assembly in the apparatus for measuring ionic activity, it has been possible to measure potential differences easily and rapidly.
However, such potential differences are extremely small, namely, in the order of from several tens of milli volts to a hundred and several tens of milli volts. On the other hand, the electrical resistance of the electrodes is extremely high, sometimes more than 10.sup.8 -10.sup.9 .OMEGA.. Accordingly, impedance of the potential difference signal is so high that, any electrical noise mixing in with the potential difference signal in a circuit (or line) between where the probes contact the electrodes and the measuring circuit, can cause a great deterioration in S/N ratio of the signal, resulting in incorrect measurement. Particularly, when the number of probes is increased to measure different kinds of ions simultaneously (or successively), the deterioration of S/N ratio is likely to become marked since there may be electrical interference, or electrical noise between the circuits (or lines) from the probes to the measuring circuit.
For this reason, the probe assembly may be connected to a appropriate electric impedance lowering means, but if one electric impedance lowering means is connected to two or more pairs of probes, there tends to be great deviation of the measured potential due to the electric interference that is produced.